For kids, too much time spent in front of a computer or TV screen can lead to narrowed arteries in the back of the eyes — an early sign of high blood pressure and future heart disease, according to a new study.

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Kids who exercised an hour or more each day had wider arteries there than did children who spent a half hour or less each day being active, the study said.

And the more time spent watching TV, the narrower the arteries were, which translates to an increase in blood pressure. The arterial narrowing from each hour of screen time would lead to a 10 millimeter of mercury (mm HG) increase in systolic blood pressure (the top number on a blood pressure reading), researchers said.

For a child who is already borderline for high blood pressure, that 10-point increase can be a lot, said Dr. John Stevens Jr., director of preventive cardiology at Children's Healthcare of Atlanta Sibley Heart Center in Georgia.

"It's more evidence that you can see effects at early life of unhealthy habits," Stevens, who was not involved with the study, told MyHealthNewsDaily.

The study was published today (April 20) in Arteriosclerosis, Thrombosis and Vascular Biology: Journal of the American Heart Association.

Australian researchers surveyed the parents of 1,492 children age 6 or 7 on the number of hours their children spend being physically active and sedentary each week.

Then the researchers took digital photographs of the arteries in the back of each child's eyes to measure how wide or narrow they were. Researchers also recorded the children's height, weight and body mass index (BMI, a measure of height in relation to weight). They also averaged three separate blood pressure measurements.

On average, the children spent 1.9 hours a day being sedentary in front of a TV or computer screen, and 36 minutes a day being physically active, the study said.

Children who spent the most time being physically active (more than an hour each day) had arteries that were, on average, 2.2 microns wider than the children who spent the least time being active (less than a half hour a day), according to the study. Each hour of TV time a day was associated with an average arterial narrowing of 1.53 microns, which is associated with an increase of 10 mm HG in systolic blood pressure.

Stevens guessed that the researchers looked at the arteries in the backs of the eyes because they are the one set of arteries that can be looked at directly without a sophisticated noninvasive test.

"You have a clear window to see the health of the artery," Stevens said.

Why are narrowed arteries bad?

It's long been known that narrowing of the arteries is correlated with high blood pressure, Stevens said. But the study shows that signs of heart risks can show up even at a young age, just by being sedentary.

Narrowed arteries put people at increased risk of heart problems because the blood vessels aren't able to dilate as well in response to changes in blood flow, thereby increasing blood pressure, he said.

"When you get increased demands for flow, can they handle it? Can they stretch?" Stevens said of the arteries.

There can be other factors in arterial narrowing, such as blood pressure, cholesterol and weight, he said. But "even before kids are overweight, even before they have elevated cholesterol and blood pressure, just with the mere fact of being physically inactive there are already some of the changes with blood vessels," Stevens said. That goes to show that "they're all interrelated."